The invention relates to a travelling-wave tube having a helical delay line, i.e. a helix, disposed between an electron-beam generating system and an electron-beam receiver with the helix being supported by a plurality of retaining members, for example of rod-like configuration, made of dielectric material and extending parallel with the helix, at least one retaining rod being made of a different material than that of the remaining retaining rods. Such a tube is generally known, for example, from German Inspection Specification 2,029,093.
The retaining rods frequently are constructed of quartz as such material has a relatively small dielectric constant (.epsilon..apprxeq.3.6), and consequently makes possible a high coupling resistance. However, quartz is a relatively poor heat conductor and as a result often the quartz-mounted molybdenum or tungsten helices, even in medium power tubes, are heated on the output side to such high temperatures, from natural losses and by impinging electrons, that irregularities occur on the helix structure and the saturation power falls off appreciably. Additionally, reflection points and parasitic attenuation zones can arise at the tube output, in particularly where copper or gold plated helices are employed resulting from evaporation and metal vapour precipitation on the dielectric rods, leading to a failure of the tube.